Monitoring Rock Freezing and Thawing by Novel Geoelectrical and Acoustic Techniques
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PUBLICATIONS Journal of Geophysical Research: Earth Surface RESEARCH ARTICLE Monitoring rock freezing and thawing by novel 10.1002/2016JF003948 geoelectrical and acoustic techniques Key Points: Julian B. Murton1, Oliver Kuras2, Michael Krautblatter3, Tim Cane1, Dominique Tschofen2, • Capacitive resistivity imaging (CRI) 2,4 3 5,6 measures freezing and thawing in Sebastian Uhlemann , Sandra Schober , and Phil Watson limestone 1 2 • Microseismic events with Permafrost Laboratory, Department of Geography, University of Sussex, Brighton, UK, Geophysical Tomography, British 3 4 characteristics typical of cracking of Geological Survey, Nottingham, UK, Landslide Research, Technische Universität München, Munich, Germany, Institute of rock bridges were detected during Geophysics, ETH Zurich, Zurich, Switzerland, 5Sensor Technology Research Centre, Department of Engineering and Design, frost weathering experiments University of Sussex, Brighton, UK, 6Apollo Fire Detectors Ltd, Havant, UK • CRI and microseismic techniques offer a viable route for long-term monitoring of freeze-thaw and fracture in mountain rockwalls Abstract Automated monitoring of freeze-thaw cycles and fracture propagation in mountain rockwalls is needed to provide early warning about rockfall hazards. Conventional geoelectrical methods such as electrical resistivity tomography (ERT) are limited by large and variable ohmic contact resistances, requiring galvanic coupling with metal electrodes inserted into holes drilled into rock, and which can be Correspondence to: J. B. Murton, loosened by rock weathering. We report a novel experimental methodology that combined capacitive [email protected] resistivity imaging (CRI), ERT, and microseismic event recording to monitor freeze-thaw of six blocks of hard and soft limestones under conditions simulating an active layer above permafrost and seasonally frozen Citation: rock in a nonpermafrost environment. Our results demonstrate that the CRI method is highly sensitive to Murton, J. B., O. Kuras, M. Krautblatter, freeze-thaw processes; it yields property information equivalent to that obtained with conventional ERT and T. Cane, D. Tschofen, S. Uhlemann, offers a viable route for nongalvanic long-term geoelectrical monitoring, extending the benefits of the S. Schober, and P. Watson (2016), Monitoring rock freezing and thawing methodology to soft/hard rock environments. Contact impedances achieved with CRI are less affected by by novel geoelectrical and acoustic seasonal temperature changes, the aggregate state of the pore water (liquid or frozen), and the presence of techniques, J. Geophys. Res. Earth Surf., low-porosity rock with high matrix resistivities than those achieved with ERT. Microseismic monitoring has 121, 2309–2332, doi:10.1002/ 2016JF003948. the advantage over acoustic emissions that events were recorded in relevant field distances of meters to decameters from cracking events. For the first time we recorded about 1000 microcracking events and Received 5 MAY 2016 clustered them in four groups according to frequency and waveform. Compared to previous studies, Accepted 14 NOV 2016 mainly on ice-cracking in glaciers, the groups are attributed to single- or multiple-stage cracking events Accepted article online 15 NOV 2016 Published online 10 DEC 2016 such as crack coalescence. 1. Introduction Understanding the impacts of freezing and thawing on bedrock properties is fundamental to assessing the stability of mountain rockwalls. Remote monitoring of rockwall temperature and related geotechnical parameters is an emerging research field for geomorphology and geohazard mitigation. Geophysical tomo- graphy can contribute significantly to this field [Harris et al., 2009] by characterizing volumetric properties of rock masses over time without the limitations of point sampling in boreholes. Both electrical and acoustic methods hold promise for long-term automated monitoring, but their systematic application to mountain permafrost is still rare, partly because sensor performance under repeated freeze-thaw conditions remains poorly understood. Although temperature-calibrated electrical resistivity tomography (ERT) with galvanic sensors can image movements of freezing fronts in rock [Krautblatter and Hauck, 2007; Krautblatter et al., 2010], conventional geoelectrical methodology imposes practical limitations. The technique relies on galvanically coupled electrodes, which must be physically implanted into a substrate [Loke et al., 2013]. For permafrost applications, this may lead to substantial magnitudes and variations in ohmic contact resistances between sensors and the host rock or soil as it freezes and thaws [Krautblatter et al., 2010]. This introduces noise and can render individual sensors or indeed entire arrays unusable during the freezing periods, thus making quantitative interpretation of geoelectrical monitoring data challenging. Geoelectrical interpretation ©2016. The Authors. is complicated further where frost weathering leads to fracture and other changes in the physical properties This is an open access article under the terms of the Creative Commons of the rock. By contrast, strategic long-term thermal and geotechnical monitoring demands stable and Attribution License, which permits use, predictable sensor properties and minimal bias of monitoring data due to the sensors themselves. distribution and reproduction in any medium, provided the original work is This paper reports a novel experimental design combining capacitively coupled resistivity sensors, con- properly cited. ventional galvanic electrode arrays, and microseismic transducers to monitor noninvasively the dynamics MURTON ET AL. FREEZE-THAW MONITORING 2309 Journal of Geophysical Research: Earth Surface 10.1002/2016JF003948 Figure 1. Instrumented blocks resting on water-saturated gravel prior to initial freezing. (a) Block 1 (tuffeau) wetted through by capillary rise. No macroscopic cracks are visible. (b) Block 2 (Wetterstein limestone), with irregular wetting front extending along cracks near base of block. (c) Blocks 4 to 6 within tank 2 (seasonal freezing). CRI plate sensors are installed on the sides of block 5 and ERT galvanic electrodes in the sides of blocks 4 and 6. of freezing and thawing of moist rock subject to varying degrees of frost weathering. The capacitive methodology, referred to as capacitive resistivity imaging (CRI), uses low-frequency impedance measure- ments across permanently installed multisensor arrays [Kuras et al., 2006], emulating the well-established ERT methodology, but removing the need for galvanic contact between the sensors and frozen soils or rocks. We applied a monitoring strategy based on geoelectrical tomography to controlled laboratory experiments simulating permafrost growth, persistence, and thaw in bedrock. Frost weathering was monitored by mea- suring heave and microseismic activity and by visually observing macrocracks. Our objectives are to explain and present results about rock fracture, geoelectrical monitoring, and microseismic events and to evaluate the performance of CRI beside conventional ERT. MURTON ET AL. FREEZE-THAW MONITORING 2310 Journal of Geophysical Research: Earth Surface 10.1002/2016JF003948 2. Materials and Methods 2.1. Monitoring Strategy Our monitoring strategy relied on instrumenting multiple rock samples with geoelectrical sensor arrays in order to compare the relative merits of the ERT and CRI techniques in different settings. This allowed us to assess the influence of rock type, freezing regime, weathering status, and the performance of different cross-sample sensor geometries. Experimental control was provided by simultaneous and continuous mea- surement of vertical rock temperature profiles, surface heave and settlement, and liquid water content in all rock samples, except where there was a risk of the simultaneous measurements interfering with each other or with the geophysical monitoring. The experiments were carried out in the cold room of the Permafrost Laboratory at the University of Sussex. This facility is designed for the physical modeling of per- mafrost conditions in rock or soil by simulating the effect of different natural freezing regimes. 2.2. Rock Types and Sample Preparation The rock types used were tuffeau and Wetterstein limestone. The tuffeau is a siliceous and texturally uniform chalk of Upper Cretaceous (Turonian) age, commonly used as a building stone in France. It was selected because it is frost-susceptible and therefore prone to macrocracking [Murton et al., 2001, 2006], allowing us to evaluate its geoelectrical and microseismic response as crack networks evolved. Its physical properties are detailed in Murton et al. [2000]. Its porosity measured under vacuum is about 47%. The compressive strength varies from 0.53 to 9.24 MPa and tensile strength from 0.07 to 1.07 MPa. Four samples of tuffeau were obtained from the Lucet limestone quarries, L’Epinay Brézé, near Saumur, in the Loire Valley, France. Prior to the experiments the samples lacked visible evidence of macroscopic cracks or other forms of weath- ering (Figure 1a). The Wetterstein limestone is a relatively uniform and isotropic fine-grained (algal) limestone deposited in lagoonal back-reef conditions during the Middle Triassic (Ladinian). It represents a very tough Alpine lime- stone and forms rock faces such as the Zugspitze North Face, some more than 1000 m high. The Wetterstein limestone was selected because it represents the natural fractured bedrock on which a CRI system might be applied in